add idToken from token response

[emilesalem]

we need to access the id token in order to call google cloud endpoints

i.e

GoogleCredential credential = GoogleCredential.getApplicationDefault(); 
credential = credential.createScoped(...); 
credential.refreshToken(); 
credential.getIdToken();

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[emilesalem]

I signed the CLA

[googlebot]

CLAs look good, thanks!

[JustinBeckwith]

gentle ping @google/yoshi-java

[chingor13]

Sorry for the delay in getting to this.

I don't think adding id_token parsing to TokenResponse is the correct approach. id_token parsing is already provided by IdTokenResponse and GoogleTokenResponse from google-api-client. The TokenResponse class is a generic implementation for OAuth2 and does not need to include id_token.

I'm also unclear on your usage of this with application default credentials as the id_token is part of the OpenID Connect protocol on top of 3-legged OAuth 2.0.

For a workaround you can use GoogleAuthorizationCodeFlow